Fix NullPointerException in case a custom document is passed to W3CDom

[casid]

When we provide a custom output Document to W3CDom#convert, we receive a NullPointerException in the W3CBuilder constructor.

I've added a test that demonstrates the behavior. Here's the stacktrace from that test without the applied fix.

java.lang.NullPointerException: Cannot invoke "org.jsoup.nodes.Element.ownerDocument()" because "this.contextElement" is null

	at org.jsoup.helper.W3CDom$W3CBuilder.<init>(W3CDom.java:364)
	at org.jsoup.helper.W3CDom.convert(W3CDom.java:250)
	at org.jsoup.helper.W3CDom.convert(W3CDom.java:238)
	at org.jsoup.helper.W3CDomTest.canConvertToCustomDocument(W3CDomTest.java:199)

[casid]

Hi @jhy,

did you get a chance to have a look at this already?

It is not urgent, but it prevents us from upgrading to the latest JSOUP version without hacking the classpath.

Thank you so much!